1. Field of the Invention
The present invention is directed to an x-ray diagnostics installation of the type having an x-ray tube, a variable aperture diaphragm and an x-ray image intensifier video chain which includes an x-ray image intensifier, a video pick-up unit, a processing circuit and a playback unit, as well as to a method for operating such an x-ray diagnostics installation. The apparatus and method serve the purpose of reproducing attenuated x-ray images.
2. Description of the Prior Art
The flow density of x-ray photons in x-ray diagnostics installations of the type generally described above is usually uniform over the cross-section of the x-ray beam of the x-ray source. The flow density is dependent on the required image quality, and defines the radiation load on the patient. If, however, only one person observes the monitor (playback unit), the x-ray photon flow is utilized only at the location at which the observer is looking at the moment with foveal vision. The human eye has much less capacity for resolution outside of this small foveal region. The patient can thus be irradiated with a lower flow density in a region of the image field outside of this small region which corresponds to the foveal region of the eye. The overall radiation load on the patient is thereby reduced.
For this reason, an aperture diaphragm is disclosed in German OS 27 40 998 which permits intensive x-rays to pass there through in a central region, whereas x-rays are attenuated in an outer region surrounding the central region. In order to insure that the observation area of the observer coincides with the region of intense irradiation, an apparatus is provided which determines the direction in which the observer is looking, and causes a corresponding shift in the aperture diaphragm. Such an acquisition of the viewing direction, however, is extremely difficult in practice.
If it is assumed, however, that the principal viewing direction of an observer will lie in the middle of the x-ray image, the diaphragm could be stationarily arranged, and the full image quality would then be present in the central region. A substantially poorer image quality, by contrast, occurs in the outer region of the image field, and this portion of the region can therefore only serve for the purpose of an overview, and cannot be used for detailed diagnostic purposes. A significant disadvantage of this known technique is the difference in the image character between the central region and the surrounding field. In the surrounding field, either a substantially higher noise level is present or, if a chronological filtering for noise reduction is undertaken, a substantially more pronounced smearing of moving subjects will be present.